Focus electrode for high power electron guns



Oct. 25, 1966 T. ZITELLI ET AL 3,281,616

FOCUS ELECTRODE FOR HIGH POWER ELECTRON GUNS Filed Oct. 50, 1961 2 Sheets-Sheet 1 l I O ,9 'J/ Ki I m v g N co 3 1 {A I I 2 5 L1 LL N/ INVENTORS LOUISTZITELLI IRVING MALTZER TTORNEY Oct. 25, 1966 L. T. ZlTELLl ET AL 3,281,616

FOCUS ELECTRODE FOR HIGH POWER ELECTRON Guns Filed Oct, 50, 1961 2 Sheets-Sheet 2 v 9 Li... g i 1 v E 2) 3 0 O 0 =2 o 9 o 9 aamod asvaalw E; m g a 2; UX l Q E k J\ 8. i G (M) ifldifiO aamod INVENTORS LOUIS T. ZITELL! IRVING MALTZER TORNEY United States Patent 3,281,616 FOCUS ELECTRODE FOR HIGH POWER ELECTRON GUNS Louis T. Zitelli, Palo Alto, and Irving Maltzer, San Carlos,

Calif, assignors to Varian Associates, Palo Alto, Calif.,

a corporation of California Filed Oct. 30, 1961, Ser. No. 148,520 3 Claims. (Cl. 31338) This invention relates in general to C.W. klystron amplifiers and, in particular, to a novel high power, high frequency 'klystron amplifier adapted, for example, for use in space communications, radio astronomy, C.W. illuminators and forward scatter.

During the past few years there has been an increasing demand for higher C.W. power, for example, in the order of 50 kilowatts or more at X-band frequencies (7.125 to 8.5 kmc.). The graphic result of a recent survey by L. S. Nergaard, RCA Review, December 1960, on power limitations of microwave transmitting tubes is depicted in FIG. 4 of the drawings. This graph shows a plot E of expected power output as a function of frequency. The article by Nergaa'rd states that increases in power output have not been large and that a major breakthrough may be required to achieve order of magnitude improvements.

The klystron amplifier of the present invention has gen.- erated about one order of magnitude more average power than any known microwave device at X-band frequencies. It performance data is superimposed, by an X, on the Nergaard graph of P16. 4 and shows that an order of magnitude improvement has been obtained over the prior art. This would indicate that a major breakthrough in the state of the art has been achieved.

Some of the problems associated with increasing the power output of X-band klystrons are as follows:

In the prior art the heater element was positioned adjacent to the cathode button and heated the button to its operating temperature by radiation of heat from the heater element to the butt-on. Normally the heater element would have to operate at 700800 centigrade above the operating temperature of the cathode button due to loss in transfer of heat by radiation from the heater element to the cathode button. The 700-800 of heat lost in the radiant heat transfer would be dissipated in other parts of the emitter structure increasing the likelihood of over heating and damaging the emitter structure of the tube. One problem area in particular is overheating of the focus electrode creating an increase in thermionic emission from the focus ring which reduced the operating efficiency of the tube and may cause arcing. A reduction in the amount of heat produced by the heater element and a more efiicient means for transfering heat from the heater element to the cathode button is greatly needed.

Another problem, associated with increasing the power output of beam tubes, is due to an increase in potential difference between the anode and the cathode structure. This increase raises the probability of arcing between the focus electrode of the emitter structure and the anode. Arcing between these two members would produce sputtered material from the focus ring which would collect on and poison the cathode button and prevent it from operating properly or could stop its operation completely.

It is the object of the present invention to provide a novel high power C.W. klystron amplifier tube of high frequency which is adapted for use in high power C.W. systems.

One feature of the-present invention is a novel focus support structure useful in conducting and radiating excessive heat away from the focus electrode to greatly reduce thermionic emission between the focus ring and the anode which could cause excessive heating of the cathode.

Patented Oct. 25, 1966 Thermionic emission could also develop into arcing causing poisoning of the cathode from sputtered focus electrode material as a result of the arcing.

Other features and advantages of this invention will become apparent from a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a longitudinal partial cross sectional view of the novel klystron of the present invention,

FIG. 2 is an enlarged longitudinal cross sectional fragmentary view of the electron emitter of FIG. 1 delineated by line 22,

FIG. 3 is a graph of power output of the present klystron' as a function of input R.F. drive power, and

FIG. 4 is a graph of average power output of state of the art transmitter tube as a function of frequency.

Referring now to the drawings, the novel multicavity klystron tube of this invention comprises three main portions: a beam producing section 1 (best seen in FIG. 2) followed by a central beam interaction section 2, wherein interaction between the beam and the applied radio frequency wave takes place to produce the amplification, and a collector section 3 where the electrons of the spent beam are collected.

The beam-producing section 1 (best seen in FIG. 2) includes a substantially cylindrical hollow focus electrode 10 including a focus ring element 10' at one end, as of, for example, oxygen-free high conductivity copper. A cathode button 11, for example a Philips type B impregnated tungsten cathode, and its associated coiled heater element 12 are supported within a cylinder 22 of, for example, molybdenum, and embedded in and sintered with a refractory dielectric material 8 as of, for example, aluminum oxide or beryllium oxide. The refractory dielectric material 8 is packed in powdered form in cylinder 22, which in turn is closed off at one end by cathode button 11. These members are then fired to a high temperature in a known manner and the impacted mateterial is sintered around the heater element. A ceramic disc 7 is preferably positioned between button 11 and the refractory dielectric material 8 to prevent the heater element from accidentally coming into contact with button 11, due to thermal expansion and contraction of the heater element 12 causing the heater element to work its way through the sintered material. Other types of cathode materials could be used with heater elements embedded in a refractory dielectric material such as described above. The type of cathode used, for example, Whether it be thoriated tungsten, or an oxide coated cathode, would depend on the amount of beam current desired.

During operation cathode button 11 is heated to approximately 1100 centigrade, i.e. 1050 centigrade to 1150 centigrade. It is desirable to operate heater element 12 at a temperature as close to 1100 centigrade as possible to reduce dissipation of excessive heat within the emitter section and to reduce power requirements. Therefore, it is desirable to reduce the temperature drop between heater element 12 and cathode button 11 to as low as possible. The sintered heater described above enhances heat transfer to the button 11 from the heater element 12 because ceramic will more readily conduct thermal energy than a partial vacuum could transfer through infrared radiation. (A partial vacuum is the environment surrounding the heater element in prior art tubes.) It has been found that the temperature of the sintered heater element operates at approximately 300 centigrade above the 1100 centigrade cathode button operating point, thus reducing heat output of the heater by 400500 centigrade over prior art cathode heater elements.

Cylinder 22 and its associated elements are in turn supported within focus electrode in axal alignment with a flared entrance 13 of a centrally apertured anode electrode 9 (see FIG. 2), formed in a transverse header of a non-magnetic material, copper for example, and attached to pole piece 14 of magnetic material. The central aperture in the anode 9 is disposed in registry with the aligned drift spaces and drift tube members within the beam interaction section 2 of the tube.

A hollow cylindrical focus electrode support 18 of a low thermal expansion material such as Kovar, supports focus electrode 10 through the intermediary of a metallic sleeve 15 which is brazed around the outside of one end of focus electrode support 18 and inside of the end of a focus electrode 10 opposite from the cathode emitter button 11. Sleeve 15 is preferably of nickel so that a good thermal conduction joint such as that formed by brazing may be made between focus support 18 and focus electrode 10.

A heat insulator cylinder 23 of, for example, tantalum, is closely fitted on the interior of focus electrode support 18 and extends almost the entire length of the focus electrode 10 in a spaced-apart manner to solidly support heater 12 and cathode 11 in position. Heat insulator 23 also acts to prevent heat from leaving the cathode emitter 11 by conduction, as heat insulator 23 is of a poor thermal conductor material and a conductor of electrical current.

A high voltage insulator is formed by a ceramic cup member 25 and supports the focus electrode support 18 from the tubes central body portion in a vacuum tight manner through the intermediary of an apertured metallic cup member 19 of a Kovar. Focus electrode support 18 is mounted within a flanged aperture center in the bottom wall 19 of cup 19.

A pair of insulated heater leads 27 are connected to ends of heater element 12 via heater rod 29 of a good electrical conductor material and supported within the focus electrode 10 and electrode support 18 in a vacuum tight manner by centrally apertured spaced-apart ceramic discs 30. The last two discs 30 are secured to focus electrode support member 18 in a vacuum tight manner. An insulator sleeve 31 fits over heater rod 29 to maintain vacuum integrity within the interior portions of the tube. The third ceramic disc 30, positioned within focus electrode support 18 at the base of the cylindrical heat insulator 23, is utilized to aid in the prevention of vibration damage in heater rod 29. Heater rod 29 is tied to heater element 12 adjacent an access hole 23 in heat insulator cylinder 23 near the heater element. The return path for the heater current is through heat insulator 23, focus electrode support 18, lead extension to heater lead 27. A pair of getter leads 38 are provided in a vacuum tight manner through apertures in ceramic disc 30 to aid in the evacuation of the tube, if necessary.

A silicone rubber end cup 16, of the type claimed and disclosed in US. Patent No. 2,990,495, for Thermionic Tube, is applied surrounding the high voltage insulator and around the focus electrode support 18, ceramic disc and heater leads 27 by a cavity molding process to reduce physical and electrical breakdown in this portion of the tube.

The above described portion of the emitter section of the present invention is attached to pole piece 14 by a pair of ring-like low thermal expansion metallic members 28 as of, for example, Kovar secured at one end to the high voltage insulator 25 and at the other end to pole piece 14, all joints being vacuum tight. A ring-like metallic shield 32 is positioned in a spaced-apart manner between the ceramic-to-metal joint of the ceramic cup 25 and ring member '28 and the focus electrode 10 to prevent arcing between the ceramic-to-metal joint and the focus electrode 10.

Thermal energy collected by the focus electrode 10 from the cathode heater is removed therefrom through conduction of thermal energy flowing along the copper focus electrode 10 away from focus ring 10. Heat conducted along focus electrode 1!) is radiated olf focus electrode 10 thereby allowing a continuous cooling process of the focus ring it). It is extremely important that focus electrode 10 be made of a material which is an excellent thermal conductor, as of a conductivity greater than 0.5 cal/(sec. cm. K.), at 850 K. to maintain a rapid flow of heat away from focus ring 16.

Heretofore in tubes of this type the focus ring was made of stainless steel for example, which could withstand high temperatures. However, these high temperatures led to high thermionic emission from the focus ring 10 to the anode 9 and also led to a complete electrical breakdown (arcing) between focus ring 10' and the anode 9 under the impressed high voltage therebetween of 25 kv.50 kv.

Focus ring thermionic emission was reduced through the use of the copper focus support. The copper ring 10 now operates at approximately 300 centigrade due to the good heat removal of support 10, while a stainless steel focus ring would operate at approximately 700800 centigrade. However, another advantage of the use of copper focus ring over stainless steel or other metals used in prior art is that arcing between a stainless steel focus ring and the anode caused stainless steel to be sputtered ofi the focus ring by the ion bombardment of the focus ring from the anode. This sputtered stainless steel material was poisoning the emitter button, causing low performance from the cathode emitter and in some cases causing the tube to cease operating altogether.

The use of copper will still not halt arcs altogether, because of the high potential drop from the focus ring to the anode. However, it has been found that the sputtered material from the focus ring being of copper will not poison the cathode emitter button. Any arcing which does occur between the focus ring and the anode is stopped in short order through the use of a high speed arc protection circuit (not shown). This circuit should have a reaction time of less than 5 microseconds.

To provide coupling of radio frequency energy into and out of the amplifier, the described interaction section 2 is cut away in its side portion for the reception of conventional waveguide sections 45 which communicate with the first and last resonator cavities through bored iris openings. Suitable R.F. windows 47 (see FIG. 1) of a wave permeable material, such as alumina ceramic are positioned at the end of waveguide sections 45 within suitable flange assemblies 48. A suitable pinch-off tube 49 is placed in gas communication with the waveguide to provide a means for evacuating the tube.

A waveguide obstacle is formed in the output waveguide 45 by a pair of thin, abrupt reactive vanes 51 and 52 positioned between the output cavity resonator and the output window 47.

It is noted that in the present tube /2 watt radio frequency drive power is sufiicient to drive the tube to slightly less than 24 kilowatts at an efiiciency of 38%. The tube is designed to operate over a limited tuning range of plus or minus 30 me. about a frequency in the X band (7.125 to 8.5 kilomegacycles).

FIG. 3 is a graph showing a family of gang curves. Curve C is taken with all the cavities synchronously tuned to the drive frequency and the remaining curves taken for various amounts of third cavity detuning. The graph shows power output as a function of drive power under operating conditions of beam voltage, 23 kv. and beam current 2.7 amps. Several of the manufactured tubes have been tested with a OW. RF. power output of 41 and 43 kw. This in an improvement over state of the art X-band OW. tubes in the order of one magnitude.

Since many modifications and variations in the described arrangement can he obviously made without departing from the scope of the invention, it is intended that all matter in the foregoing description or shown in the accompanying drawing should be interpreted as illustra tive and not in a limiting sense.

What is claimed is:

1. A. high power, hi-gh voltage electron gun assembly including, means forming an electron emitting member for emitting electrons to form a high power electron beam with an excess of kilowatts of average beam power, means forming an apertured anode spaced from said emitter means for accelerating the emitter electrons through the aperture in said anode, means forming an insulator disposed between said anode means and said emitter means for holding off an applied voltage in excess of 15 kv., an elongated hollow cylindrical focus electrode encircling said emitter means in spaced apart relation from said anode and operated i-n use near emitter potential for focusing the beam through said anode, said focus electrode being made of copper whereby heat radiated from said emitter means and collected by said focus electrode is conducted evenly along the length of said elongated focus electrode to facilitate radiation and conduction of thermal energy from said focus electrode permitting said focus electrode to operate at a temperature below which thermal emission from said focus electrode may not readily occur and whereby copper which is sputtered from said focus electrode onto said emitter will not poison said emitter means.

2. The device according to claim 1 wherein said hollow elongated focus electrode has one free end portion thereor" defining a focus ring disposed between said anode and emitter and the remaining portion including a fixed end portion defining a support means, said emitter having an emitter button, a heater element, and a heat insulator support member for supporting said emitter button substan- 6 tially from the fixed end of said elongated focus electrode and extending axially internally thereof in a spaced-apart relation to said focus electrode.

3. The device according to claim 2 wherein said heat insulator support member is of tantalum.

References Cited by the Examiner UNITED STATES PATENTS 2,569,872 10/1951 Skehan 313-32 2,798,182 7/1957 Costa 315-340 2,817,784 12/1957 Katz 313-340 2,834,949 5/1958 Duffy 333-98 2,852,715 9/1958 Rich 315-5.48 2,861,213 11/1958 Dahnan 313- 2,888,599 5/1959 Jepsen 315-539 2,892,115 6/1959 Rudnick 313-85 2,931,942 4/1960 Strand 315-39 2,932,755 4/1960 Jeppson 315-534 2,939,036 5/1960 Nelson 315-537 2,944,187 7/1960 Walter 315-39 2,946,918 7/1960 Crapuchettes 315-39 3,007,292 11/1961 Duran et a1 313-340 3,008,063 11/1961 Bernstein et al. 313-24 3,047,324 7/ 1963 Salisbury 315-539 3,104,338 9/1963 Symons 313-24 3,125,701 3/1964 Mann 313-340 DAVID J. GALVIN, Primary Examiner.

BENNETT G. MILLER, Examiner. 

1. A HIGH POWER, HIGH VOLTAGE ELECTRON GUN ASSEMBLY INCLUDING, MEANS FORMING AN ELECTRON EMITTING MEMBER FOR EMITTING ELECTRONS TO FORM A HIGH POWER ELECTRON BEAM WITH AN EXCESS OF 20 KILOWATTS OF AVERAGE BEAM POWER, MEANS FORMING AN APERTURED ANODE SPACED FROM SAID EMITTER MEANS FOR ACCELERATING THE EMITTER ELECTRONS THROUGH THE APERTURE IN SAID ANODE, MEANS FORMING AN INSULATOR DISPOSED BETWEEN SAID ANODE MEANS AND SAID EMITTER MEANS FOR HOLDING OFF AN APPLIED VOLTAGE IN EXCESS OF 15 KV., AN ELONGATED HOLLOW CYLINDRICAL FOCUS ELECTRODE ENCIRCLING SAID EMITTER MEANS IN SPACED APART RELATION FROM SAID ANODE AND OPERATED IN USE NEAR EMITTER POTENTIAL FOR FOCUSING THE BEAM THROUGH SAID ANODE, SAID FOCUS ELECTRODE BEING MADE OF COPPER WHEREBY HEAT RADIATED FROM SAID EMITTER MEANS AND COLLECTED BY SAID FOCUS ELECTRODE IS CONDUCTED EVENLY ALONG THE LENGTH OF SAID ELONGATED FOCUS ELECTRODE TO FACILITATE RADIATON AND CONDUCTION OF THERMAL ENERGY FROM SAID FOCUS ELECTRODE PERMITTING SAID FOCUS ELECTRODE TO OPERATE AT A TEMPERATURE BELOW WHICH THERMAL EMISSION FROM SAID FOCUS ELECTRODE MAY NOT READILY OCCUR AND WHEREBY COPPER WHICH IS SPUTTERED FROM SAID FOCUS ELECTRODE ONTO SAID EMITTER WILL NOT POISON SAID EMITTER MEANS. 